Fix thread safety, redundant async/await, and section-count logic from review feedback

[Copilot]

Addresses five valid review comments on the metrics/structured-results PR.

Changes

• CacheService — CacheHits/Misses/Skips were plain auto-properties, unsafe for concurrent task access. Replaced with private int backing fields; writes use Interlocked.Increment, reads use Volatile.Read.

• NewsletterService session hooks — Hooks were declared async solely to await Task.CompletedTask before returning. Dropped the async/await Task.CompletedTask pattern in favour of Task.FromResult<T?>.

// Before
OnErrorOccurred = async (input, invocation) => {
    logger.LogWarning(...);
    await Task.CompletedTask;
    return new ErrorOccurredHookOutput { ErrorHandling = "retry" };
},

// After
OnErrorOccurred = (input, invocation) => {
    logger.LogWarning(...);
    return Task.FromResult<ErrorOccurredHookOutput?>(new ErrorOccurredHookOutput { ErrorHandling = "retry" });
},

• GenerateCopilotNewsletterAsync — Added explicit await Task.WhenAll(newsSectionTask, releaseSectionTask) before extracting results via await, making the concurrent intent clear and improving timing accuracy.

• DoctorCommand — Removed a redundant second CopilotClient instantiation and ping; PrintCopilotStartupStatusAsync already creates a client, pings, and surfaces failures via warnings.

• CountSections — Replaced unconditional +1 (assumed a welcome section always exists) with a scan of pre-heading lines; only counts a welcome section if non-whitespace content is actually present before the first ##/### heading.

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